When ships such as tankers, large cargo ships, etc. sail with no oil or cargoes or a small amount thereof being loaded, generally they sail with ballast water stored in a ballast tank to ensure the stability and balance of the ships. Usually, the ballast water is loaded thereinto by pumping up, for example, seawater at the port of discharge and is discharged at the port of loading. Thus, since, for example, seawater of the port of discharge is used for the ballast water, the ballast water contains, for example, aquatic microorganisms living around the port of discharge, and the aquatic microorganisms are discharged together with the ballast water at the port of loading.
Methods for treating ballast water are described in, for example, Patent Document 1 and Patent Document 2. Patent Document 1 discloses an electrolysis-type apparatus for treating ballast water that is provided with an electrolytic cell for electrolyzing seawater, a circulating pump for connecting a ballast tank and the electrolytic cell, and a control unit that controls the supply of direct-current voltage to the electrolytic cell and that controls the circulating pump in such a manner as to adjust the hypochlorous acid concentration in the ballast water to annihilate marine microorganisms inside the ballast tank. Patent Document 2 discloses an electrolytic sterilizing apparatus for ship ballast water, wherein the electrodes of the first-group electrode set, which come into first contact with flowing water, are plate-shaped electrodes and the other-group electrode sets are mesh-shaped electrodes.